Method of making pellets



July 17, 1951 R. 5. HOOD METHOD OF MAKING PELLETS Original Filed July19, 1943 2 Sheets-Sheet l July 17, 1951 R. s. HOOD METHOD OF MAKINGPELLETS 2 Sheets-Sheet 2 iginal Filed July 19, 1943 zeaz ia .9 if *1 5ifloa ney Patented July 17, 1951 METHOD OF MAKING PELLETS Ralph S. Hood,Marblehead Neck, Mass., assignor to Monsanto Chemical Company, St.Louis, Mo., a corporation of Delaware Original application July 19,1943, Serial No.

495,384. Divided and this application Decembe!" 26, 1947, Serial No.793,852

3 Claims.

This invention relates to the pelleting of finely divided solidsubstances, and particulary to continuous processes for pelleting suchmaterials on a tonnage basis.

In the past it has been common to form pellets of finely divided solidmaterials by feeding into a rotating drum or cylinder the solid materialto be pelleted and a liquid, which can be either water or a solution ofor a molten mass of the substance being treated. The solids and liquidssupplied to the drum were usually fed in at a point above the rotatingmass, and as a result the material adjacent the liquid feed tended toagglomerate and stick to the walls of the drum in-the form of a more orless plastic mass, thereby markedly reducing the efficiency of theoperation. The pellets produced in this manner are also usually quitevoluminous and bulky with the result that it was not possible to pack aslarge a quantity of the material in the shipping containers as wasdesired. Also they are usually quite friable and rough surfaced whichtends to produce dust during handling.

It is accordingly a primary object of the invention to produce pelletsfrom finely divided solids in a manner which does not result in undueagglomeration or sticking of the solids to the walls of the pelletingapparatus.

A further object of the invention is to produce pellets which arerelatively smooth and dense and which do not tend to form dust insubsequent handling of the material.

A further object of the invention is to produce in the drum pellets ofrelatively uniform size and to avoid the formation of excessively largepellets, whereby it is unnecessary subsequently to classify the pelletsor to interrupt the process to remove oversized pellets.

A further object of the invention is to greatly increase the efiiciencyand output of the usual type ofrotary pelleting drum and to adapt suchequipment for continuous operations.

Still further objects and advantages of the invention will appear fromthe following description and appended claims when considered inconnection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view, partly in section, of a pelletingapparatus which is equipped with means for recirculating and otherwisetreating' the pelleted material.

Fig. 2 is a sectional view of the pelleting drum or-cylinder taken onthe line 2-2 of Fig. 17 The invention is carried out in general by firstsupplying to a suitable pelleting drum or cylinder, such as the drum l0shown in the drawings, a bed ll of finely divided solids of thesubstance to be pelleted, and then rotating the drum at such a speedthat the material is carried up the side of the drum almost to the top,whereupon it falls in such a way as to produce a waterfall effect.Preferably this operation is carried out in such a manner as to producea relatively hollow region or a region A of low concentration (see Fig.2), in which the particles are sparsely arranged and in which thecirculation of the particles is partially restricted, that is, in whichthe particles remain during the greater portion of the actual pelletformation before moving into the outer, faster moving region B of themass. A liquid, such as water, an aqueous solution of the material beingtreated, or a molten mass of this material, is then supplied to the drumby feeding it substantially into the relatively hollow or partiallyrestricted region referred to above. If the relative amount of solidsand liquid is properly adjusted, a relatively plastic zone is formed, inwhich the particles and liquid gradually agglo-merate and form pelletswithout contacting the walls of the drum. Thus caking of the material onthe drum walls is avoided and the eniciency of the operation is greatlyincreased. Moreover, by forming the pellets in a relatively plasticzone, much smoother and stronger pellets are formed which are alsosubstantially dust free.

In carrying out the methods of this invention itis possible to use anysuitable drum or cylinder having a restricted opening at one or bothends", and arranged to rotate on a horizontal or substantiallyhorizontal axis. In general, it is preferable to tilt the drum somewhatfrom the horizontal, usually not higher than about 30, as this increasesthe capacity of the drum and permits an improved classification of thepellets. Thus, by adjusting the angle of the inclination of the drumaxis, it is possible to remove selectively from the drum primarily thelarger pellets being formed. This also prevents the formation ofexcessively large pellets which otherwise tend to remain in the drumandinterfere with efiicient operation of the apparatus.

In some instances in carrying out the methods of this invention it isunnecessary to resort to any further treatment in order to obtainpellets of the desired characteristics. In other instances, however, itis necessary to cool, heat, dry or otherwise treat the pellets. Forexample, when molten material is used as the liquid feed, it is usuallynecessary to cool the mixture being pelleted, while when chemicalreaction takes place prior to or during the pellet formation, it isfrequently necessary to heat the mixture to initiate or foster thereaction. Drying is usually required in those instances where a solutionof the substance being pelleted is employed as the liquid portion of thefeed.

Any suitable means may .be used to accomplish plished by controlling theamount of materials supplied, the ratio of solid to liquid feed, theposition of the liquid feed pipe in the drum and the speed of rotationof the drum, and in those instances where classification is desired, byalso adjusting the angle of inclination of the axis of the drum.

The particular apparatus shown in Fig. l of the accompanying drawings isprimarily useful in pelleting processes in which finely divided solidsare mixed with molten material and in which the maintenanceofpredetermined temperature conditions in zone A of the drum is required,i. e. temperatures considerably below the freezing point of the materialbeing pelleted.

Thus, although the material in zone A is maintained in a relativelyplastic state as compared -with the remainder of the material in thedrum, the capacity of the drum is increased if the temperature in zone Ais maintained well below the freezing point, In the case of phthalicanhydride,

for example, the temperature of zone A is preferably held at a point C.or more below the freezing point. Although reference is made sole- 1y tothe treatment of phthalic anhydride in describing the operation of theapparatus shown, it

should be understood that the invention is adapted to the treatment ofother materials, and that in such cases it is merely necessary to resortto common expedients in either heating, cooling or drying the material,as required.

Referring to Fig. 1, the numeral Hi represents a rotary drum of the typewhich is closed at one end and terminates at its other end in afrustrated cone l2 having an opening 13 of sufficient size to permit thefeeding of the material to be treated and the egress of the pelletsresulting from the rotation of the drum. Drum I1! is supported at itsclosed end by the rotary shaft I4, which in turn is supported by theU-shaped bracket or other suitable support I5. 'Rotation is obtained bymeans of the sprocket l6 and any suitable source of power (not shown).The phthalic anhydride to be pelleted is fed into the drum, preferablyin finely divided form, by means of the screw conveyor ll, while themolten phthalic anhydride is applied by means of the pipe 18 from anysuitable source of supply (not shown). The pellets formed as a result ofthe mixing and rotating action of the drum gradually spill over into thehopper l9, and are cooled upon passing through the screw conveyors 2nand 2|,

which may be jacketed as indicated at 22 to per- -mit the application ofany suitable cooling medium. The conveyors 20 and- M are connected bythe pipe 23 to permit continuous flow of the pellets from one conveyorto the other, and conveyor 2i terminates at its upper end in a shoot orhopper 2t which feeds the pellets onto the screens 25 and 26, where theyare separated from the fines. The pellets caught on the screen 26 aredelivered as finished product to the barrel or other suitable receptacle21, while the pellets on the screen 25 are passed through the grindingmechanism 28 and returned to the pelleting drum through the pipe 29along with the fines which drop down through the pipe 30 to the conveyorll. In starting the apparatus, a sufiicient amount of phthalic anhydrideis fed into the drum ill, after being ground in the grinder 28, to fillthe drum about half full. This initial bed i l of finely dividedphthalic anhydride is then rotated in the drum at such a speed that theanhydride tends to ride up the upmoving side wall of the drum and fallover much in the manner of a waterfall. By rotating the material in thismanner a relatively hollow or sparsely filled region A of partially re.-stricted movement (see Fig. 2) is formed in the rotating mass. Themolten anhydride is then supplied by means of the pipe !8, which isinserted into the rotating mass of solids at about the middle of theregion A. By proceeding in this way it is possible to maintain the pipeIt in a predetermined position without the rotating mass tending to moveit or snap it oif its supports. Moreover, the molten material is fed toa slow moving or partially restricted region, in which the particlesremain, more or less as in an eddy current, during a large part of thepelleting oper. ation before moving into the outer faster moving regionB of the mass. This permits the mass to become plastic and to form thepellets slowly with the result that they are harder and smoother thanpellets formed in ordinary pelleting operations. As previouslyindicated, control of the temperature of the rotating mass in the drumin is obtained by cooling the pellets outside of the drum and returninga portion of them together with the fines to the drum. Temperaturecontrol may be obtained in other ways, however, as for example, bymerely controlling the relative amounts of liquid and solid feed,without recirculation, or by using a jacketed drum and supplying asuitable cooling medium thereto. If desired, all three of thesemethods-may be used to obtain the desired cooling or temperature'control, namely, recirculation, cooling of the drum, and adjustment ofthe ratio of hot molten and cool solid feeds.

Although the pelleting drum shown in the drawings is of the type whichis closed at one end, other types of drums may be used, if desired,including drums open at both ends. It is also not always essential touse a conical-shaped end construction, although inclined walls at theexit end of the drum facilitate the egress of the pellets and theselective removal of pellets of a relatively uniform size range.Classification of the pellets in the drum itself is controlled furtherby adjusting the angle of inclination of the drum axis from thehorizontal; but it is also possible to obtain good results, either withor without classification, in a horizontally arranged drum.

- to provide suitable means for cooling the pellets and preparing themfor return to the pelleting drum, as by grinding. A separate grindingap- 'paratus is not always necessary, however, as in certain substances,for example reactive mixtures of sodium chloride and sodium acidsulfate, heating of the pellets is required instead of cooling. In usingthe apparatus shown it is then only necessary to apply a heating mediumto the jackets of the conveyor 2!) and/or 2! instead of a coolingmedium. Likewise, if the recirculating equipment is omitted and ajacketed drum is used instead, a heating medium is employed instead of acooling medium. In still other instances, as in the case of ferricsulfate, in which a ferric sulfate solution is used as the liquid feed,the pellets are dried instead of being heated or cooled, and in suchcases the recirculating equipment is arranged to include a dryingmechanism, such as any conventional rotary "vention involve the mixingof a liquid and a finely divided solid substance in the rotary drum.However, it is also possible to supply two or more solid substances tothe drum, one of which upon being heated melts and then coats the othersolid substance or substances. The meltable or liquefiable substance isfed into the drum at a point within zone A, thereby forming hard andsmooth-surfaced pellets on rotating the drum in the manner hereinbeforedescribed. It is also possible to employ two or more solid substances,one of which melts upon being heated and then reacts with one or more ofthe other solid substances being fed to the drum. While reaction takesplace the molten liquid formed simultaneously mixes with the solidmaterial present to form pellets. For example, sodium acid sulfate maybe mixed with sodium chloride 1 in a rotary drum, such as the drum Ill,while simultaneously heating the mixture. The acid sulfate soon melts,and reacts with the sodium chloride to form hydrogen chloride gas andsodium sulfate. liquid particles present in the mixture form pellets dueto the rotary action of the drum on the mass, which pellets upon leavingthe drum are completely converted to sodium sulfate. In carrying out aprocess such as the above, the two solid substances may be supplied tothe drum either as a previously prepared mixture or in separate feedpipes or conveyors. In either case, however, the conveyor or conveyorsshould be arranged to discharge into the partially restricted zone A inorder to obtain the advantages of this invention.

It is also possible to supply to the drum finely divided seed materialalong with two or more liquids, which liquids react upon mixing to forma solid substance of the same composition as the seed material. Prior toreaction the liquids agglomerate the seed material, and upon completionof the reaction the liquid portion of the agglomerates solidify, therebybinding the seed During the reaction the solid and particles into theform of relatively hard and smooth pellets.

In some instances, as for example, when the material to be pelleted issufliciently friable to form fines by attrition in the rotating drum,and the liquid employed is a hot molten liquid, the process may becarried out continuously without the addition of new fines or therecirculation of a portion of the pelleted product as fines, i. e. bycontinuously supplying only the hot molten liquid to the drum andsimultaneously cooling to solidify the liquid about the fine particlesalready present or resulting from attrition. In general, however, it ispreferable to supply continuously along with the liquid being used aproportionate amount of finely divided solids to provide the nuclei foradditional pellets, since in most cases insuifioient breakage of thepellets takes place in the drum to provide enough fines for practicaloperation. Moreover, the processes of this invention usually tend toform pellets of relatively hard and smooth surfaces, which do notreadily break up to form fines. Regardless, however, of the physicalstate of the materials which are continuously supplied to the drum, itis essential in order successfully to carry out this invention to startthe process with an initial bed of finely divided solids in the drum andto direct the liquid feed, or the material which becomes liquid duringthe process, into the drum at a point below the surface of the rotatingmass in the drum, preferably at a point within the hollow or more orless sparsely filled region of restricted circulation referred to.

By proceeding in accordance with the various methods described above itis possible to obtain discrete pellets of generally round shape, whichare relatively dense and hard and have an exceptionally smooth surface.The pellets consist of agglomerates of relatively small particles whichhave been bonded together by solidification of the liquid employed orformed during the process. For example, in the case of phthalicanhydride the pellets consist of agglomerates of relatively smallparticles of phthalic anhydride bonded together by means of phthalicanhydride material which has been solidified from a molten state. In allinstances, the pellets are characterized by being free flowing and freefrom a tendency to form dust under normal conditions of handling anduse.

Where reference is made herein and in the appended claims to finelydivided solids, it is to be understood that this has reference to eithera powdered material or to particles of larger size, which arenevertheless smaller than the pellets it is desired to produce.

This application is a division of my copending application Serial No.495,384, filed July 19, 1943, now Patent No. 2,436,771, granted February24, 194.8. The present application covers the use of two or moredifferent solid substances at least one of which forms a liquid uponbeing heated, while the applicants Patent No. 2,436,771 covers the useof a solid and a liquid.

What is claimed is:

1. The method of continuously pelleting finely divided solids whichcomprises initially supplying a mass of such solids to a rotary drum ofthe type which rotates about an axis not inclined more than about 30from the horizontal, continuously rotating said drum at such a speedthat the mass of solids rides up the upmoving side wall of the drum tothe upper portion thereof and falls over inwardly, thereby forming arelatively sparsely filled region'of relatively restricted circulationin the central portion of the rotating 'mass, continuously supplying atleast two difformed.

2. The method of continuously pelleting finely divided solids whichcomprises initially supplying a mass of such solids to a rotary drum ofthe type which rotates about an axis not inclined more than about 30from the horizontal, continuously rotating said drum at such a speedthat the mass of solids rides up the upmoving side wall of the drum tothe upper portion thereof and falls over inwardly, thereby forming arelatively sparsely filled region of relatively restricted circulationin the central portion of the rotating mass, continuously supplying atleast two different solid substances to said drum, at least one of whichsubstances forms a liquid upon being heated and reacts with another of'said substances, and simultaneously heating said substances, whereby atleast one of said substances forms a liquid and said substances react,said solid substancesbeing fed intosaid drum at a point below thesurface of the rotating mass and within the region of relativelyrestricted circulation, whereby smooth-surfaced and relatively hardpellets are formed.

- 3. The method of continuously forming pellets of sodium sulfate whichcomprises initially supplying a mass of finely divided sodium sulfate toa rotary drum of the type which rotates about an axis not inclined morethan about 30 from the horizontal, continuously rotating said drum atsuch a speed that the mass of solids rides up the upmoving side wall ofthe drum to the upper portion thereof and falls over inwardly, therebyforming a relatively sparsely filled region of relatively restrictedcirculation in the central portion of the rotating mass, continuouslysupplying sodium chloride and sodium acid sulfate to the drum, andsimultaneously heating the said substances to cause liquefaction of thesodium acid sulfate and inter-action between said substances, saidsubstances being fed into said drum at a point below the surface of therotating mass and within the region of relatively restrictedcirculation, whereby smooth-surfaced and relatively hard pelletsconsisting substantially of sodium sulfate are formed.

RALPH S. HOOD.

I REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 870,746 Pennock Nov. 12, 19072,436,766 Davis Feb. 24, 1948 2,436,771 Hood Feb. 24, 1948

1. THE METHOD OF CONTINUOUSLY PELLETING FINELY DIVIDED SOLIDS WHICHCOMPRISES INITIALLY SUPPLYING A MASS OF SUCH SOLIDS TO A ROTARY DRUM OFTHE TYPE WHICH ROTATES ABOUT AN AXIS NOT INCLINED MORE THAN ABOUT 30*FROM THE HORIZONTAL, CONTINUOUSLY ROTATING SAID DRUM AT SUCH A SPEEDTHAT THE MASS OF SOLIDS RIDES UP THE UPMOVING SIDE WALL OF THE DRUM TOTHE UPPER PORTION THEREOF AND FALLS OVER INWARDLY, THEREBY FORMING ARELATIVELY SPARSELY FILLED REGION OF RELATIVELY RESTRICTED CIRCULATIONIN THE CENTRAL PORTION OF THE ROTATING MASS, CONTINUOUSLY SUPPLYING ATLEAST TWO DIFFERENT SOLID SUBSTANCES TO SAID DRUM, AT LEAST ONE OF WHICHSUBSTANCES FORMS A LIQUID UPON BEING HEATED, AND SIMULTANEOUSLY HEATINGSAID LIQUE-